Do not try to create /dev/efi device node before devfs is initialized.
Split efirt.ko initialization into early stage where runtime services
KPI environment is created, to be used e.g. for RTC, and the later
devfs node creation stage, per module.

Switch the efi device to use make_dev_s(9) instead of make_dev(9). At
least, this gracefully handles the duplicated device name issue.

Remove ARGSUSED comment from efidev_ioctl(), all unused arguments are
annotated with __unused attribute.

Reported by: ambrisko, O. Hartmann <ohartman@zedat.fu-berlin.de>
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks